1. Field of Invention
This invention relates to a heat transfer process for the heating or cooling of a fluid by means of direct contact with an intermediary, immiscible liquid enhanced by the presence of a semi-buoyant, surface active media bed.
2. Description of Prior Art
Heat transfer processes have been an essential component of human activity since prehistory. The first heat transfer process utilized by mankind was in the use of sunlight for body warmth. With the development of controlled fire, heat; transferred from an open fire, was used to cook food which presented a more palatable and hygienic food format. To prevent the charring associated with cooking over open fires, hot rocks taken from open fires, were eventually utilized to provide cooking surfaces and heat sources for better controlled cooking. Today, heat transfer processes are employed in all phases of human activity. Examples of such are cooking, space heating and cooling, fabrication, warfare, transportation, generation of light, preservation of food, medicinal care, chemical conversion processes, to name only a few.
There are three basic modes of heat transfer. These modes are radiative, convective and conductive. All heat transfer applications employ one or more of these modes.
Radiative heat transfer occurs as electromagnetic energy emitted from a thermal source, is absorbed by a thermal sink. This energy induces molecular vibration in the absorbing matter, which is observed as heat. Radiative heat transfer is the only heat transfer mode in which heat can be transferred from a thermal source to a thermal sink across open space. Heat transfer through radiation was illustrated in the foregoing paragraph as the use of sunlight to warm and thereby transfer heat to the body. The emitting thermal source being the Sun and the absorbing thermal sink being the body.
Convective heat transfer is a mode in which matter, heated from a thermal source, physically transports (convects) heat from the thermal source to a thermal sink. Typically this convecting matter is a gas, liquid or plasma (referred hereafter as a fluid). Convective heat transfer incorporates three steps. The first step entails direct contact heating of a convecting fluid by a thermal source. The second step involves the transport of the heated convecting fluid away from the thermal source. The third step occurs when heat is transferred from the transported convecting fluid into a thermal sink by means of direct contact between the two. Convective heat transfer was exemplified in the foregoing examples as the use of open fire to cook food. In that example, gases, as the convecting fluid, were heated by direct contact with the fire, rose and carried (convected) heat away from the fire. Cooking ensued as a result of direct contact between the food and these convecting gases.
The third heat transfer mode is conduction. Conduction is the process whereby heat is transferred internal to or between two or more contacting matter bodies. Essentially Conduction is the mechanical equalization process of molecular vibratory energy in or between contacting matter.
If a matter body is heated unevenly so as to induce uneven temperatures within the body, and if the matter body cannot support internal fluidized convection or internal cavity radiative transfer then heat will naturally transfer by means of conduction from warmer to cooler sections of the body. This effect is governed by the geometry of the matter body, temperature gradients within the matter body and the thermal conductivity (a physical property) of the material comprising the body.
If two or more matter bodies of differing temperatures are brought into direct contact with each other, heat will be transferred by means of conduction from the warmer into the cooler of the bodies. This effect is governed by the geometry of both the contacting matter bodies and the contacting surfaces, temperature gradients within the bodies, the temperature differential between the bodies and the thermal conductivity of the materials comprising the matter bodies. Heat transfer by means of conduction was illustrated in the foregoing examples as the use of hot rocks to provide heat for cooking. By means of conduction, heat is transferred from the interior to the surface of the rock where it is used for cooking.
All three heat transfer modes are currently employed in industry with conductive and convective processes being predominant. Transference of heat using both convective and conductive modes is common. In a typical application, heat is transferred from a thermal source, through the walls of a tube and into a liquid contained within the tube. The transference of heat from the thermal source to the exterior surface of the tube is generally a convective process. The transference of heat from the exterior surface through the tubing wall to the interior tubing wall surface is a conductive process. The transference of heat from the interior tubing surface to the liquid is generally a convective process. One objective of such an application would be to alter the chemical, thermodynamic or phase conditions of the liquid. An example of such an application would be the heating of water for a phase conversion to steam. Another objective would be to heat and employ this liquid as a heat transfer medium. In such an application the heated liquid would be transported by means of the tubing to a remote location where the entrained heat is either discharged or employed for process. An example of such an application would be electric power plant cooling in which circulating water transports heat from a steam condenser to a cooling tower or radiator for environmental discharge.
Conductive heat transfer processes in which heat is transferred across a solid wall, as discussed above, are common. In such applications the solid wall provides both the medium for heat conduction as well as for mechanical separation of the materials or events on opposite sides of the wall. Heat conduction through the solid wall is controlled by the thermal conductivity of the material of the wall, the temperature difference across the wall and the thickness of the wall. Wall material with low thermal conductivity and/or excess thickness impedes heat transfer. Wall material with high thermal conductivity and/or thinness enhances heat transfer.
Conductive heat transfer through layered walls is controlled by the relative thickness and thermal conductivity of the materials comprising the layers. Heat is transferred through the layers in series. As a result of this series configuration, heat transfer is governed by the least conductive of the layers. A layer comprised of material of low thermal conductivity can substantially reduce heat transfer rates through the wall.
Many liquids when heated (or cooled) experience chemical or physical changes which result in the precipitation or formation of solids. These solids can accumulate on surfaces which are in contact with the liquid. Heat transfer into (or out of) such a liquid by means of direct contact between the liquid and a heated (or cooled) wall surface usually results in the accumulation of solids on the heated (or cooled) wall surface. The buildup of such precipitates and solids on the wall generates a layer through which heat must be transferred to heat (or cool) the liquid. The precipitates and solids comprising this layer generally have low thermal conductivity. This layer impedes heat transfer. Heat transfer rates through the wall and into (or out of) the liquid can be reduced to unacceptable levels as the layer deposits and thickens. In the parlance of the heat exchange equipment industry, this accumulation process and the resulting negative effects are referred to as scaling or fouling of the heat exchanger or heat exchange surfaces.
Scaling and fouling impair heat transfer because of the buildup of thermally resistant materials on the heat transfer wall surfaces. In contrast, corrosion and chemical attack thins, pits, cracks and generally reduces the mechanical integrity of heat transfer walls. This problem manifests itself not in the imposition of heat transfer but rather in reduction of the service life of heat transfer equipment. Corrosion and chemical attack are generally provoked by incompatibility between the fluid being heated or cooled and the materials of construction of the heat transfer equipment. Such attack can also be incited by chemical additives intended for the reduction of scaling and fouling. Typically, problems associated with corrosion and chemical attack of heat transfer equipment are resolved through the use of different materials of construction and/or chemical treatment of the fluid to buffer the offending chemistry.
Scaling and fouling of heat exchange surfaces is a prevalent problem of industry. In many industries the labor and costs associated with mechanical and/or chemical cleaning of heat exchangers to remove scale and fouling represents a formidable financial burden. Various methods have been employed to minimize fouling and scaling of solid wall heat exchangers. Chemical additives to modify pH, surface tension characteristics or other chemical parameters are sometimes used to reduce precipitation or other depositional tendencies of the liquid.
Another method has been the incorporation of self cleaning mechanisms to continually or periodically scrape or abrade fouling and/or scaling materials from heat transfer surfaces. This method, generally known as a scraped wall heat exchanger, is often used for those circumstances where the scale or fouling material is valuable and is the desired end product of the heat transfer process. A related method with the additional advantage of providing convection enhancing turbulence is described in U.S. Pat. No. 4,616,698, granted to Klaren. This method incorporates a fluidized granular mass suspended in a liquid undergoing heat transfer. This granular material contacts the heat exchange solid walls, abrades deposits and generates turbulence within the liquid.
A third common method, particularly in those applications for which the liquid is circulated for the transport of heat, is discharge (blowdown) of the fouling liquids and recharge (makeup) with less fouling or fresh liquid. The blowdown carries some of the fouling materials away from the heat transfer process. The makeup then dilutes the remaining liquid to maintain the fouling and scaling materials in solution and reduce their tendency toward deposition.
Direct contact, immiscible, liquid to liquid heat transfer has been postulated and seen some limited applications. This process is advantageous in that there are no solid walls through which heat is transferred. The lack of such walls eliminates the possibility of fouling, scaling or corrosion of heat transfer surfaces and thereby assures efficient heat transfer and acceptable equipment life. The prior art focus of direct contact immiscible, liquid to liquid heat transfer has been to transfer heat from fouling, hot brines into immiscible fluids, generally liquid hydrocarbons, which show little affinity for water. As examples, of such applications the reader is referred to U.S. Pat. No. 4,167,099, entitled Countercurrent Direct Contact Heat Exchange Process and System in which the inventors Wahl and Boucher describe a direct contact heat exchange process using a plurality of stages to contact a working fluid, such as a hydrocarbon with hot geothermal brines. Another similar patent, granted to Sheinbaum, reference U.S. Pat. No. 3,988,895 discloses a power generation process whereby a working fluid such as isobutene is heated through direct contact with a hot brine. U.S. Pat. No. 4,089,175, granted to Woinsky describes a similar process with the significant difference of specifying that the direct contact heat transfer process occur within the confines of a contacting tower maintained at a pressure equal to or in excess of the critical pressure of the working fluid. pentane is introduced as a supercooled fluid into direct contact with a heated fouling and scaling prone liquid brine. Other related patents are U.S. Pat. No. 1,905,185, granted to Morris and U.S. Pat. No. 3,164,957 granted to Fricke. The direct contact heat exchangers of prior art, as discussed in the foregoing, employ contacting vessels containing solid, essentially immobile sieves, trays or packing.
In the prior art, a typical application uses isobutane as the working fluid. Isobutane being less dense than the brine, rises through the brine and is heated by means of direct contact with the hot brine. As the isobutane is heated it changes phase to a vapor. This vapor exits from the top surface of the brine and is passed through demisting equipment and utilized to extract work by means of a Rankine (or other) thermodynamic cycle or employed for process heating.
A less common technique for heat transfer with scaling, fouling or corrosive liquids is by means of non-solid wall convective and radiative heat transfer processes. An example of a convective process is direct contact heating of a liquid by bubbling hot gases through it. This process, referred to as submerged combustion, has seen some limited use. A related process, in which superheated steam is injected into an aqueous based liquid is also used.
Another technique for the heating of scaling, fouling or corrosive liquids is by means of radiative heating. This technique has been used for the heating of liquids amenable to radiative absorption. A familiar example of such is the use of microwaves for the heating of aqueous based liquids.
Heating (or cooling) of fouling, scaling or corrosive liquids currently and historically has presented serious and expensive difficulties. Present solutions to reduce these problems suffer from several disadvantages:
(a) Resolution of corrosion problems through the use of more compatible materials of construction is generally burdened by the high cost and/or low thermal performance of such materials.
(b) Chemical buffering of corrosive liquids often is employed. However, cost and undesirable contamination of the liquid being heated or cooled frequently renders this approach unacceptable.
(c) Chemical treatment to reduce the fouling and scaling tendencies can be quite expensive. Many of the required chemicals are somewhat exotic and must be tailored to the specific liquid application. Often these chemical costs are excessive and a substantial financial burden to the user.
(d) Chemical treatment must be tailored to specific liquids. Often the efficacy of the treatment is dependent upon specific liquid constituents, pH, temperature or other characteristics, which may vary. The occurrence of such variance often reduces or impedes the effectiveness of the chemical treatment. This can result in fouling, scaling and consequential damage and/or expense.
(e) Chemical treatment can generally only provide limited protection. Often chemical treatment is used only to extend operating times between cleaning. Cleaning operations are still required to maintain the heat transfer efficiency.
(f) Chemical treatment often requires the use of harsh chemicals with high tendencies for corrosion or other damaging processes of metallic heat transfer surfaces. To mitigate the effects of these tendencies, heat transfer surfaces must often be manufactured of exotic, expensive and often difficult to fabricate materials. These corrosion resistant materials often present a compromise over ideal heat exchange material which would comprised of a material chosen for thermal conductivity rather than corrosion resistance. This compromise reduces the efficiency of the heat transfer process and necessitates the application of larger, more expensive heat exchangers.
(g) Often liquid discharge (blowdown) and fresh recharge (makeup) are concurrent with chemical treatment. In such cases the chemical treatment is employed primarily to minimize required discharge and recharge volumes. The discharge liquids often contain residues of the chemical additives. These residues can be hazardous, rendering the discharge volumes difficult to treat, handle or discard.
(h) Mechanical self cleaning (scraped wall) and granular abrading heat exchangers are expensive, often complicated and susceptible to mechanical failure.
(i) As a result of scraping and abrasion, the composition of the heat transfer surfaces incorporated in scraped wall heat exchangers must be hard and/or relatively thick. Often the required composition is exotic and expensive. Additionally, the composition is often a compromise over ideal heat exchange wall material which would be thin and comprised of a material chosen for thermal conductivity rather than abrasion resistance. This compromise reduces the efficiency of the heat transfer process and necessitates the application of larger, more expensive heat exchangers.
(j) Control of scaling and fouling by means of fouling liquid discharge and fresh liquid recharge often presents difficulties relative to the handling, treatment or disposal of the fouling liquids. Discharge treatment costs, environmental considerations and recharge liquid costs are inherent problems to this approach.
(k) Control of scaling and fouling by means of fouling liquid discharge and fresh liquid recharge require monitoring of the liquid properties to maintain the proper discharge and recharge rates. Excursions from this control can result in excess costs and liabilities if the rates are too high and fouling, scaling and potential damage if the rates are too low.
(l) Direct contact, immiscible fluid to liquid heat exchangers demonstrate limited efficiencies as a result of affinity and agglomeration of the direct contacting fluids. Typically, the immiscible fluid is dispersed as droplets into the scaling and fouling prone liquid. Droplet heat transfer rate is dependent upon the surface area of the droplet and the thermal gradient surrounding the droplet.
Surface tension effects result in droplets which are generally spherical in shape. The surface area to volume ratio of a sphere is 1/r, where r is the spherical radius. As a result of this inverse proportionality, larger drops in a dispersed volume generate smaller surface areas. Heat transfer from (or into) the dispersed droplets is regulated by the surface area of the droplets. Because of the affinity of like fluids, the dispersed droplets agglomerate as they pass through the scaling and fouling prone liquid. This agglomeration effect increases the size of the droplets which reduces the dispersed surface area and, as a result, the heat transfer rate.
Heat transfers from hot to cold. The impetus for this transfer is the temperature differential or more precisely the temperature (thermal) gradient perpendicular to the surface of transfer. The rate of heat transfer through any given surface is regulated not only by the area of the surface but also the temperature (thermal) gradient present at the surface. Heat transfer rates into or out of the surface of a droplet are regulated by the thermal gradient present at the surface. The thermal gradients affecting a droplet are controlled by the temperature difference between the droplet surface and the surrounding liquid, and the radius of the droplet. For a spherical surface, the thermal gradient is inversely proportional to the spherical radius. As the spherical droplets agglomerate and increase in size, the thermal gradients are reduced and the impetus for heat transfer diminishes. The consequence is also a reduction in heat transfer rates as the droplets agglomerate and increase in size.
The natural agglomeration of the dispersed droplets in a direct contact, immiscible fluid to liquid heat exchange process results in reduced heat transfer rates with the resulting loss of overall process efficiency. Dispersion plates and trays have been employed in an attempt to breakup the agglomerating droplets but have proven to be troublesome due to plugging, fouling and scaling of the plate and tray surfaces. Direct contact, immiscible, fluid to liquid heat exchange processes have demonstrated few applications because of these inefficiencies.
(m) Direct contact submerged flame type heat exchangers are capital intensive and require considerable energy to bubble the hot gases through the liquid to be heated. The hot gas is typically placed into the lower end of a liquid contacting column and is released to bubble upward, in direct contact, through the liquid column. The heat transfer occurs as the bubbles rise.
To permit adequate heat transfer, it is necessary to provide sufficient direct contact time between the hot gas bubbles and the liquid. The upward velocity of the bubbles is generally high, therefore the contacting column must be tall to insure sufficient contacting time for heat transfer. The hot gas is injected into the bottom of the liquid column. For injection to occur, the hot gas pressure must be in excess of the hydrostatic pressure of the column.
The thermal energy content of a heated gas bubble rising through the liquid is small. For adequate heat transfer, plentiful volumes of hot gas must be contacted with the liquid. The high volume, high pressure and compressibility of the hot gas exacts a large measure of operating energy and expense for the direct contact, submerged flame heat transfer process.
(n) Direct contact submerged flame type heat exchangers generally require pollution control equipment such as drift and/or mist eliminators. This equipment can be expensive and troublesome. Submerged flame combustion vapor products exhaust aggressively from the top of the heated fluid. Carryover of liquid and particulates in this exhaust stream are difficult to control. Plugging and cleaning maintenance of the pollution control equipment as well as environmental liabilities are significant problems with direct contact submerged flame heat exchangers.
(o) Submerged flame type heat exchangers generally must use high grade heat such as that generated through the combustion of fuel. The low thermal conductivity of the bubbling gas inhibits the heat transfer rate into the liquid. The heat transfer impetus is the temperature differential between the bubble and the surrounding liquid. Bubbles comprised of high temperature gas are preferable to offset the low thermal conductivity effect. The exhaust or flue gas resulting from combustion of fuel is typically used for the bubbles because of the associated high temperature. This process is both expensive, since high grade heat in the form of fuel combustion is employed, and prone to contamination of the heated liquid with combustion byproducts.
(p) Submerged combustion processes are difficult to maintain if particulates are forming in the heat exchange process. The gas bubbles rising through the liquid generate high turbulence which inhibits settling of particulates. The particulates remain entrained in the liquid. For removal of the particulates the submerged flame process is terminated long enough for the particulates to settle. The settled solids are removed and the submerged flame process reinstated. The time required for the settling and solids removal operation varies with application but is always a burden on the process.
(q) Direct contact heating by means of steam injection is applicable only under those circumstances for which contamination by the steam condensate is acceptable. Such applications are generally limited to those examples where the heated liquid is aqueous based and open processes where the steam condensate or the mixture of condensate and heated liquid is discharged for disposal or other use.
(r) Radiative heating has found limited application because of capital and operational expense, liquid radiative absorption characteristics and energy inefficiency. Radiative heating requires that the liquid being heated absorbs the radiated energy. Often the liquid to be heated is transparent and radiative heating of the liquid is not possible.
The source of radiation is a high temperature thermal source as is generated by electric element resistance heating, fuel combustion or electromagnetic generation. All of these processes generate wasted heat which is convected or conducted away from the process and lost. Liquids which can absorb radiative energy for heat transfer generally do so over a limited wavelength band. Radiation outside of the limits of this band is not used and is wasted.
(s) Microwave heating of aqueous based liquids is a common place occurrence in many households and commercial eating establishments. This process works well for heating on a relatively small scale and where energy efficiency is not a concern.